This invention relates generally to cable television systems, and more specifically to taps, which are used in such systems.
A communication system 100, such as a two-way cable television system, is depicted in FIG. 1. The communication system 100 includes headend equipment 105 for generating forward signals that are transmitted in the forward, or downstream, direction along a communication medium, such as a fiber optic cable 110, to an optical node 115 that converts optical signals to radio frequency (RF) signals. The RF signals are further transmitted along another communication medium, such as coaxial cable 120, and are amplified, as necessary, by one or more distribution amplifiers 125 positioned along the communication medium. Taps 130 included in the cable television system split off portions of the forward signals for provision to subscriber equipment 135, such as set top terminals, computers, and televisions. In a two-way system, the subscriber equipment 135 can also generate reverse signals that are transmitted upstream, amplified by any distribution amplifiers 125, converted to optical signals, and provided to the headend equipment 105.
A cable television tap 130 that splits the forward signal is depicted in FIG. 2. The tap 130 includes an input port 205 for receiving the signal, an output port 210 for passing the signal through to other portions of the communication system 100, and subscriber ports 215, each of which couples a portion of the signal to a subscriber. A tap 130 commonly includes four, eight, or sixteen subscriber ports 215, although an even larger number of subscriber ports 215 can be included if necessary. There are also access ports 225 on the tap housing 220 for cable television system technicians to access seizure screws 230. The seizure screws 230 are a mechanical connection and an electrical conductor between a center conductor of the coaxial cable to a printed circuit board of the tap 130. Depending on installation of the tap 130, the input and output ports can be interchanged with the relative access ports 225.
The design specifications on the distribution taps should meet the IEEE standard (587), which refers to the specification of a combination surge that a product should be designed and rated to meet or exceed. More specifically, the taps should withstand 6 kilovolts (kV) of peak voltage and 3 kiloamps (kA) of peak current for a specified amount of time. The conventional tap circuitry is not rated to withstand a combination surge equal to, or in excess of, this IEEE standard, which could, potentially, result in product failure and loss of signal to the next device in the communication system 100 if a voltage equals or exceeds this standard.
One example of a combination surge that affects the tap in a cable television system is a voltage surge along a power company""s transmission power lines. The cable television systems typically utilize power company""s transmission poles for hanging the communication medium, e.g., coaxial cable, on which the taps will be spliced. Alongside the cable television system coaxial cable is generally a high voltage wire that transmits power supplied by the power company to consumers. Generally, the cable television systems lease these lines from the power company; therefore, prudence suggests that the power company""s rules and regulations should be followed. One regulation that the cable television service providers should practice is a grounding procedure of the communication system 100 onto the power company""s transmission ground.
One disadvantage of this practice is the power company grounding may not be adequate enough to protect the cable television system""s devices. A sudden voltage surge throughout the transmission power lines to a transmission ground may, as a result of poor grounding, travel into the communication medium of the cable television system 100. This voltage surge causes a spike in current, which may potentially exceed the component ratings of the surrounding taps, resulting in failure of the tap or a disruption of service to the consumer of the cable television system 100.
Another example of a combination surge is a lightning strike that causes a voltage spike on the communication medium of the cable television system 100. A lightning strike may occur anywhere throughout the communication system 100; therefore, if there is not adequate protection for the electrical and passive devices, e.g., taps 130, this surge in power may affect the circuitry in the surrounding equipment.
In light of some of the disadvantages associated with power surges throughout a cable television system 100 and the more recent expectation of meeting the IEEE standard (587), there is an increasing expectation of cable service providers that the design and performance of products, e.g., distribution taps, will be reliable and surge-resistant. It is also important that any such features be easily retrofitted into existing products with an ease of installation to keep upgrade costs at a minimum. Thus, what is needed is a protective device for the distribution taps that will limit the combination voltage and current surges to within the ratings of the components used within the taps.